The long-term goal of this research is improve quality of life post-stroke by improving the development and prescription of rehabilitation efforts. This proposal is focused on gaining fundamental knowledge about neuromuscular mechanisms that impair gait and balance post-stroke and lead to increased risk of falls and decreased mobility. This is important because rehabilitation paradigms for retraining gait and balance have been limited by a lack of understanding of the underlying neuromuscular mechanisms leading to functional impairments. Therefore, this project will examine how muscle coordination is impaired across both walking and balance post-stroke. Further, we will also examine how muscle coordination is changed after a successful gait rehabilitation program. In Aim 1, we will identify and compare impairments in muscle coordination during balance and gait in individuals post-stroke. In Aim 2, we will examine mechanisms of improved muscle coordination through a novel gait rehabilitation program. We will record muscle activity using electromyography (EMG) while walking at self-selected speed and during reactive balance (responses to support-surface translations while standing). We will characterize muscle coordination using motor modules, which identify groups of muscles that are activated together. A reduction has been shown post-stroke in the number of motor modules in the paretic leg during gait that is related to lower scores on clinical measures of both gait and balance. In many of these individuals, the reduced number of motor modules was due to abnormal coupling of the ankle plantarflexors with the hip and knee extensors. Even in those individuals without this abnormal coupling, recruitment of the ankle plantarflexors were often inappropriately timed. We do not know if this inappropriate timing or abnormal coupling also occurs during balance. We expect that the number, composition and timing of motor modules on the paretic leg will be altered due to abnormal muscle coupling and inappropriate recruitment timing in both gait and balance. We will also take advantage of an ongoing rehabilitation program as a platform to understand how neuromuscular mechanisms underlying the control of balance and gait can be improved. This rehabilitation program targets improving ankle muscle recruitment through functional electrical stimulation and has been shown to improve biomechanical and clinical measures of gait and balance performance. However, muscle activity has yet to be examined and thus it is unknown if these improvements are related to restoration of appropriate ankle activity and/or whole-limb muscle coordination. We expect that this rehabilitation program restores appropriate whole-limb muscle coordination during both gait and balance. The results of the proposed project will help us to understand how abnormal muscle coupling and inappropriate recruitment timing leads to functional impairments in gait and balance post- stroke and will contribute to the understanding of how rehabilitation ameliorates these functional impairments, driving the improvement and specificity of such interventions to improve outcomes and quality of life.